Untersuchung elektronischer Zustände in unmagnetischen Einkristallen mit spinaufgelöster Photoemission

Garbe, J.

Book/Report

FZJ-2018-03058

Untersuchung elektronischer Zustände in unmagnetischen Einkristallen mit spinaufgelöster Photoemission

Garbe, J. (Corresponding author)FZJ*

1988
Kernforschungsanlage Jülich, Verlag Jülich

Jülich : Kernforschungsanlage Jülich, Verlag, Berichte der Kernforschungsanlage Jülich 2228, 114 p. (1988)

Please use a persistent id in citations: http://hdl.handle.net/2128/18584

Report No.: Juel-2228

Abstract: Electronic states in nommagnetic single crystals have been investigated using spin-resolved photoemission with circularly polarized light. By normal emission experiments from the highly symmetric Pt(111) surface with normally incident light it was possible to verify theoretical predictions for the spin polarization from the symmetry of the electronic wavefunctions and the relativistic dipole selection rules. Covered by major peaks in the intensity spectra weak transitions into a second final state band were present, which could be identified only by the spin detection. For one peak a dramatic decrease of the transition matrix element at the hybridization point of two final state bands was found. Only in the polarization data the corresponding transition could still be detected at higher photon energies. The experimentally determined energy of the hybridization point is consistent with a suggestion of Wern et al. /1985/ to shift the ground state bands above the Fermi level upward by 0.7 eV as an approximation for realistic excited states. Using the same scattering geometry experiments were performed at a surface with relatively low symmetry, the Pt(110) surface. In the (110)- or $\sum$-direction the spin polarisation can not be predicted by group theoretical methods. According to the relativistic selection rules a non-zero polarization may only be measured if hybridization of wavefunctions with different symmetries is present in either the initial or the final states. Experimental values of up to 30% prove the existence of strong hybridization mainly in the initial state bands. By a calculation of the symmetries of the valence electron wavefunctions with an LCAO-model most features of the polarization spectra could be explained qualitatively. The data provide a quantitative test for realistic wavefunctions from self-consistent band structure calculations. At room temperature the clean Pt(110) surface is reconstructed showing a (1X2)-structure (in thermodynamical equilibrium). Experiments at a clean and a CO-covered unreconstructed sur face show that the spin polarization is not sensitive to the surface structure and thus may be interpreted in terms of direct bulk transitions. A non-disperging peak close to the Fermi energy exhibiting high intensity but no spin polarization is only present at the reconstructed surface. It is tentatively attributed to emission from a high density-of-states localized at the surface. At the TaC(100) surface the first spin- and angle-resolved photoemission experiments at a binary system with circularly polarized light of variable photon energy were done. A slightlylower spin-orbit-splitting of 0.45 eV was observed as compared to 0.6 eV at platinum. Low intensity structures showing definite spin polarization may not be explained by direct bulk transitions. They are interpreted as emission from defect states correlated with carbon vacancies. Energetically they are located approximately 1.2 eV above the spin-orbit-split es-bands. An additional positively polarized peak close to the Fermi energy may be due to a surface resonance. Further experiments at the Pt(111) surface were done to investigate spin-dependent intensities of electrons emitted into a direction away from the surface normal. The scattering of excited electrons at the surface depends on their spin-state via spin-orbit-coupling, and, therefore, so do the measured intensities in the vacuum. The polarization of the excited electrons is inverted by a change of the light helicity, and an intensity asymmetry may be defined as the normalized difference of intensity spectra obtained with positive and negative helicity light. Maximum asymmetries of more than 40% were found in certain emission directions. An antisymmetric behaviour with respect to a crystal mirror plane was observed in agreement with theory. To a limited extent such experiments provide a possibility to qualitatively determine the spin polarization (sign) without the laborious explicit spin detection.

Contributing Institute(s):